246 The American Geologist. October, 1S97 
THE PHYSIOGRAPHIC DEVELOPMENT OF THE 
UPPER MISSISSIPPI VALLEY. 
By Oscar H. Hershey, Freeport, 111. 
The Cretaceous period was characterized, on the site of the 
present Mississippi valley, by the most favorable conditions 
of widespread baseleveling. The land, never greatly elevated, 
stood at a nearly permanent level with relation to the sea, 
during the entire time, enabling the streams to so widen their 
valleys and surface erosion to tear down the divides, that the 
entire land area, long before the close of the period, had as- 
sumed the attitude of a low-lying, very slightly undulating 
plain, rising gently from the sea coast to the most distant in- 
terior parts. Over the outcropping areas of limestones and 
soft shales and sandstone, which then underlaid the surface 
from the granites and quartzites of the central area of Wis- 
consin and northeastern -Minnesota, to the Archean belt (now 
partly buried under later strata) which lies south of the south- 
ern Appalachian and Ozark provinces, this Cretaceous pene- 
plain must have been one of a remarkably even character. 
During the later or upper Cretaceous time, the sea on the 
west was gradually encroaching upon this great baseleveled 
plain. Possibly slight warpings along the coast or the marine 
erosion of small sea-clilfs, enabled the streams near their em- 
bouchure into the sea, to excavate slight channels, not suffic- 
ient however, to materially interfere with the plain-like char- 
acter of the land surface. As the Cretaceous waters took 
possession of wide stretches of nearly level land, they depos- 
ited fine sediment in thin strata, first in the slight valley 
troughs, and then upon the general flat surface of the inter- 
stream portions. Since the close of the period and the dis- 
appearance of the sea from the Mississippi basin, subaerial 
erosion, supplemented in the north by glacial abrasion, has 
generally removed the soft Cretaceous strata from the areas 
where they never existed in great thickness. But here and 
there an isolated outlier remains in the shallow depressions, 
indicating the former extent of marine strata of this age. By 
this means, the geologists of the Minnesota survey have been 
enabled to determine that at the time of maximum submerg- 
ence in the Cretaceous period, it is doubtful if any large body 
